So i wonder what happens if you quest succesfully in first stage of "Hunt for Gollum" reveal cards (per stage effect) and add something like Treacherous Fog - there is now more threat than willpower. I imagine you still place progress as if that card was not there yet (even tho placing progress occurs after determining succesful/unsuccesful).

Also i looked at the thread about Eleanor and Surge and Doomed stuff and did not found the answer - if you discard card with Eleanor effect does it still resolve its Surge and Doomed? I thought so, cos they are not under when revealed (unless they are, like "if there is no cards in staging area this card gain surge") but apparently some people said that Surge and Doomed resolved after the when revealed effect and when this time comes card is already discarded.

Also, if Lego kills, lets say troll in journey along the Anduin or some other thign that prevents from going to next stage and you have already max progress tokens on it, can his effect trigger and place progress on new stage, cos quest stage immediately replaced? Can this be done if his effect finished stage and then he have Blade of Gondolin, can blade's effect be put on next quest stage?

The extra cards happen after you quest successfully, so you place progress before adding them.

Doomed and Surge go off regardless of Eleanor (Or Test of Will, for that matter).

And yes, both those Legolas things would work, because quest advancement is a passive effect which happens instantly when the conditions are met, cutting in before you have a chance to trigger your responses (Also simultaneous effect timing for the Blade).

A precision about your first question:

In fact, a Forced effect triggers as soon as it's requirements are fulfilled, in this case, as soon as the player(s) have quested successfully. It means that the effect resolves before physically placing progress tokens on the quest; so if you would reach 8 progress on the stage 1B on this turn, thus completing the stage, you would still apply the forced effect (draw 3 encounter cards, choose 1 and stage it) before progressing to stage 2A.